Many different types of polyurethane-based adhesives have been previously developed for particular applications or uses and polyurethane prepolymers have become increasingly popular for use in these polyurethane-based adhesives. Polyurethane prepolymers are typically prepared by reacting an excess of a diisocyanate compound with a polyhydroxy compound to form a "prepolymer" which contains free isocyanate groups. The free isocyanate groups of the polyurethane prepolymer can then be crosslinked or cured as desired by exposure to moisture, the use of various crosslinking agents, or the like.
U.S. Pat. No. 4,623,709 discloses the preparation of polyurethane prepolymers having a low amount of unreacted diisocyanates by reacting an asymmetrical diisocyanate with a polyhydric alcohol to form a reaction product which is then combined with a symmetrical diisocyanate. Various polyhydric alcohols mentioned for use in reaction with the diisocyanate compound include OH-functional polyesters, OH-functional polyethers, and short chain diols, triols, and tetraols.
U.S. Pat. No. 4,891,269 discloses a method of employing an adhesive containing a film forming ethylene-vinyl copolymer, a polyurethane prepolymer, a polymeric tackifier and antioxidant. The polyurethane prepolymer is prepared from aromatic polyisocyanates and a polyhydroxy compound having a hydroxyl number and molecular weight sufficient to obtain an appropriate viscosity. Various polyhydroxy compounds mentioned for use in forming the prepolymer include polyoxyalkylene compounds, polybutadiene diols, hydroxyl-containing polyesters, hydroxyl-containing polyester amides, and polyalkylene ether glycol compounds.
U.S. Pat. No. 4,675,237 discloses an adhesive layer used in the manufacture of laminated glasses. The adhesive layer contains a cold polymerizable polyurethane formed from an isocyanate component comprising a monoisocyanate with a double ethylene bond and from a polyol component and at least one polymerization initiator. Various hydroxy compounds described for use as the polyol component include polyetherpolyols, polyesterpolyols, polycaprolactones, polycarbonatepolyols, polyesterpolycarbonatepolyols, and polybutadienes with hydroxyl or carboxyl functionality.
U.S. Pat. No. 3,642,943 discloses a solution of polymeric material which contains a mixture of an isocyanate-containing urethane prepolymer and a copolymer of an acrylic monomer and an adduct of an organo diisocyanate and a hydroxyl alkyl acrylic monomer. The isocyanate-containing urethane prepolymer is prepared by reacting an organo diisocyanate and a polyol or a mixture of polyols. Various polyols described as being useful in forming the urethane prepolymer include polyetherpolyols prepared by the addition polymerization of ethylene oxide and/or propylene oxide and a polyol like trimethylol propane. Other polyols mentioned include polyesterpolyols prepared by copolymerizing a low molecular weight polyol with a polycarboxylic acid.
U.S. Pat. No. 4,112,017 describes radiation curable coating compositions based on unsaturated addition polymerizable urethane resins. The urethane resins are prepared from a polyiisocyanate compound, an unsaturated addition polymerizable monomeric compound, and a polyester polyol. The polyester polyol is the reaction product of 1,3-butylene glycol or neopentyl glycol, adipic acid, isophthalic acid, and an aliphatic polyol having at least three hydroxyl groups.
U.S. Pat. No. 4,133,723 discloses energy curable compositions based on at least one unsaturated urethane resin. The resin is the reaction product of a polyisocyanate compound, a poly(alkylene oxide)polyol, and an unsaturated addition polymerizable monomeric compound. The resin is utilized in combination with a reactive diluent system, an aromatic photosensitizer, and an aromatic photoinitiator.
It has been found that many of the previously known polyurethane adhesives based on polyurethane prepolymers such as those described above do not fare well when applied to opaque, transparent or translucent materials such as window shades or various outdoor fabrics that are exposed to high heat, high humidity, and prolonged natural or artificial UV radiation. Specifically, traditional polyurethane prepolymer-based adhesives have a tendency to discolor, depolymerize, and/or hydrolyze upon prolonged exposure to environmental conditions such as high heat, high humidity, and sustained UV radiation. Additionally, many of the previous polyurethane prepolymer-based adhesives are not capable of selectively bonding thin layers of materials such as sheer fabrics without diffusing or blocking onto adjacent layers of fabric not intended to be bonded. This tendency to diffuse or block onto other layers of fabric creates significant problems when attempting to bond only selected layers of a multi-fold fabric or the like.
A need therefore exists for a polyurethane adhesive that can effectively be applied to fabric-like materials to produce adhesive bonds that will withstand high heat, high humidity and sustained UV radiation. The need for this type of adhesive is particularly strong in industries that produce transparent or light colored fabric-like materials such as window shades which are continuously exposed to sunlight, high temperatures and/or humidity.